Robert Holmes was tasked with writing a second serial for the season (having already written The Ribos Operation, the season opener) after Ted Lewis' Shield of Zarak was not completed. Holmes was asked to create a monster of epic proportions. This worried many crew members, due to cost restrictions. The Power of Kroll was to be the fourth story of the season until it was swapped with The Androids of Tara, originally fifth.

The K9 prop would not work on swampy ground and so does not appear in this story. However, John Leeson appears in the role of Dugeen. He took the part after the actor originally cast became unavailable.

Contents

The Doctor and Romana arrive on the marsh moon of Delta Magna in search of the next segment of the Key to Time. They are caught in the conflict between the native Swampies and the crew of a chemical refinery. The presence of a gun runner complicates matters; to make things worse, the Swampies intend to awaken Kroll, the giant god that lives beneath the swamps.

In the far future on one of the moons of Delta Magna sits a vast methane and protein collection and manufacturing refinery. The staff of five — the bullying leader Thawn, his technical support, Fenner, Harg, and Dugeen, and a hovercraft pilot — are in charge of the operation. A green-skinned humanoid native called Mensch, one of the Delta Magnans, serves them.

These servile people once lived on the planet but were moved to the moon when the Earth Empire identified Delta Magna as commercially valuable. Sadly for the green-skinned natives, known as Swampies, this is now also the case on the moon. Thawn grows concerned about the possibility of insurrection when he learns that the environmentalist group Sons of Earth has taken an interest in the Swampies and the gun-runner Rohm-Dutt is rumoured to be journeying to the moon. Thawn and Fenner go outside the refinery to see if he has arrived, suspecting a mysterious spacecraft which has touched down nearby is Rohm Dutt’s.

The Doctor and Romana I have also arrived on the moon, searching for the penultimate segment of the Key to Time. Because of the swampy terrain, K9 is confined to the TARDIS — or "marooned", as Romana puts it. However, the Tracer they are using to locate the segment is giving some very strange readings that makes their quarry difficult to pinpoint.
They are ambushed by Swampies. Romana is taken away with Rohm Dutt and a Swampie patrol. The Doctor is found by Fenner and Thawn and returned with them to the refinery. There he displays a bewildering familiarity with methane catalysation, with the refined protein shot into space for delivery to Delta Magna. The Doctor is also intrigued by the position of Mensch, whom the refinery workers do not even regard as a properly sentient being. The Doctor slips away to try to find Romana, but she has got herself into trouble. The Swampies have tied her to a rock and are about to sacrifice her to Kroll, a giant squid creature that lives under the swamps, which they worship as a god.

Kroll appears from the waters. It is a vast squid – or rather a Swampie dressed as one. The Doctor unmasks the fake monster and rescues Romana.

Back at the refinery, Dugeen has detected a huge creature some two miles wide inside the swamp. The Doctor and Romana have found reference to the real Kroll in a Swampie testament. It says the enormous creature ate a previous Swampie High Priest and has made three appearances. A fourth is due soon.

Rohm-Dutt has supplied the Swampies with weapons but when they are used against Thawn, who is searching the swamps for the Doctor, they are defective and explode whenever fired, killing one of the Swampies. Seconds later, a huge tentacle erupts from the water and seizes Mensch, dragging him to his death. In the confusion, Rohm-Dutt calls out to Thawn; his cries go unnoticed. Everyone's eyes are drawn to the vast, squid-like creature that rises from the swamp, towering above the horizon. Kroll is very real, very large and very hungry. Ranquin tries to mollify it with prayer and Kroll disappears beneath the swamp's surface. Despite Ranquin's belief that Kroll has spared his believers, the monster has simply failed to notice the Swampies and it is soon drawn to the refinery. One of its tentacles enters a pipeline and soon smashes its way inside, dragging crewmember Harg to his death.

The gun-runner is taken as a prisoner to the Swampie camp. The Doctor and Romana arrive and introduce themselves to the tribe and its leader, Ranquin. The elder decrees all three "dry-foots" will die by the seventh ritual of Kroll: a painful and cumbersome affair involving stretching the victims on creepers and wood that will eventually snap their backs. In fear, Rohm-Dutt admits Thawn paid him to sell the faulty weapons to the Swampies to discredit the environmentalist Sons of Earth movement. All three are secured and their punishment begins as a violent storm starts. The Doctor rescues himself, Romana and Rohm-Dutt with "vocal vibrations" which shatter a window above, allowing the rain to pour on the vines. This relaxes them enough for the trio to escape, pursued by the Ranquin and his tribe.

The three remaining refinery crew members decide to redirect the upcoming orbit shot against Kroll. They note the creature is on the move towards the Swampie settlement. When it reaches there, its first victim is Rohm-Dutt. The Doctor deduces that the creature hunts by vibration and uses this knowledge to keep Romana and himself safe from the tentacles. The enormous Kroll rises to the swamp's surface.

The Doctor and Romana sneak back into the refinery and are horrified; Thawn calculates the missile strike will destroy the creature and the Swampies. Dugeen is equally appalled. When he tries to stop the missiles being fired, Thawn kills him. The Doctor, however, disconnects the firing mechanism in the rocket silo. Thawn investigates and finds the two culprits, more certain than ever that they are Sons of Earth spies too. The tables are turned when the Swampies attack the breached refinery and kill Thawn. Ranquin, who has led the assault, believes the deaths of the “dry foots” will help appease Kroll, but the creature is now above the swamp again and hurling itself against the bulk of the refinery.

While Ranquin fruitlessly prays to Kroll to stop the attack, the Doctor orders all the refinery's equipment activated to try to confuse Kroll. It has the desired effect and the attack stops. Ranquin refuses to let his faith in Kroll be shaken however. Finding one of Kroll's tentacles, he begs his god to destroy the "dry foots." Instead, Kroll grabs Ranquin and drags him away to his doom. The remaining Swampies realise the creature is beyond appeasement.

It is not, however, beyond science. The Doctor has worked out that Kroll has been magnified to giant proportions by a segment of the Key to Time, disguised as a Swampie relic which the squid creature swallowed with an earlier High Priest. He uses the Tracer to eliminate the giant Kroll and retrieve the fifth segment of the Key.

The Doctor finally saves the refinery from destruction by aborting the automatically scheduled orbit shot, which would have caused an explosion. He tells Fenner, the only survivor of the refinery crew, to use his time wisely until a rescue mission arrives from Delta Magna and to try to understand the Swampie culture. The Doctor and Romana head back to the TARDIS with another segment of the Key in their possession.

The Sons of Earth are a nonviolent political movement who believe that all life originated on Earth. They want to return humanity there, though "none of them can have seen Earth". In the novelisation, the Sons of Earth believe that Delta Magna has been ruined by people such as Thawn, Fenner and Dugeen, who have, according to them, come across many worlds, filled them up with people and left to find other worlds.

Working titles for this story included Moon of Death, Horror of the Swamp and The Shield of Time. It should be noted that this final working title was part of a short-lived idea that all stories in Season 16 were going to be entitled The (Something) of Time, with the last story (which eventually became The Armageddon Factor) named simply The Key to Time.

This story features a guest appearance by Philip Madoc. Madoc had been invited to play Thawn, but the invitation was withdrawn because Neil McCarthy had already accepted the role. He agreed to play the part of Fenner when Alan Browning, who was slated to play Fenner, fell ill before the start of production.

John Leeson, best known as the voice of K9, appears in this episode as Dugeen, partly because the location of the story rendered the use of K9 unfeasible. This marks Leeson's only on-screen appearance in Doctor Who. Coincidentally, the end of the previous story, The Androids of Tara — which sees K9 stranded in a rowing boat in the middle of Castle Gracht's moat — offered a practical demonstration of why water and K9 did not mix.

The BBC's Head of Serials, Graeme MacDonald, was so unimpressed by the set designs in this story that he ordered that designerDon Giles was never to work on the series again. By coincidence, part three includes a line of dialogue in which the Doctor criticises the decor of the Swampies' execution chamber and recommends its architect be fired.

The episodes of this serial are noticeably shorter than average. While it was normal at the time for individual episodes to fluctuate between twenty-three and twenty-five minute, the episodes of this serial clocked in as little as just over twenty-one minutes. In addition, parts three and four begin with longer-than-usual reprises of the events leading up to the preceding cliffhangers, both clocking in at close to ninety seconds each. The final running time for the whole serial is 90 minutes, as is noted by the DVD of this story.

Tom Baker was credited as 'The Doctor' in Radio Times, as opposed to the usual 'Dr. Who'. The use of this billing had previously occurred with TV: Image of the Fendahl. In the closing titles, the term The Doctor would not be used until Castrovalva in 1982.

The sacrificial stone is referred to as the "Stone of Blood". An earlier story in this season was called The Stones of Blood.

The Doctor, talking to himself before his final encounter with Kroll, says he is seven hundred sixty years old. This suggests he is either rounding up (as his age was given as seven hundred fifty-nine in The Ribos Operation) or he's had a birthday since beginning the Key to Time mission. A birthday scene was originally planned for The Stones of Blood but dropped by Graham Williams, who considered it "too self-indulgent", as was noted in Doctor Who Magazine.

The green make-up for the Swampies was specially ordered from Germany. Unfortunately for the actors, the special solvent needed to remove the make-up wasn't ordered at the same time, and so they either had to undergo chemical showers at RAF Bentwaters or have their skin scoured in the hotel. Many had a green tinge to their skin for weeks.

As reported by Doctor Who Magazine, this story was a replacement for one entitled The Lords of Misrule by distinguished screen writer Ted Willis. (Ted Willis never worked on Doctor Who. Thriller writer Ted Lewis, best known for the seminal Get Carter, did work on an ultimately unused Key to Time story, title unknown.)

The Doctor, talking to himself before his final encounter with Kroll, says that he is seven hundred sixty years old. This suggests that he is either rounding up (as his age was recently given as seven hundred fifty-nine on), or he's had a birthday since beginning the Key to Time mission. (TV: The Ribos Operation)

While marooned in the TARDIS, K9 spent time playing games with the TARDIS computer. (AUDIO: Luna Romana)

It was also released with same stories as Doctor Who: The Key to Time, an extras-laden box set limited to 15,000 in its initial UK release on 24 September2007, later followed by wide release in Region 1 on 3 March2009 as The Key to Time - Special Edition.

Variations - A BBC local news programme visits the story location during filming. Includes interviews with Tom Baker and Mary Tamm.

In Studio - Raw footage taken from one evening's videotaping, focusing mainly on scenes involving Neil McCarthy, Philip Madoc and John Leeson, plus on-set discussions between Tom Baker, Mary Tamm and the director over how to play one scene.